The general principles of ICP-MS are well known. ICP-MS instruments provide robust and highly sensitive elemental analysis of samples, down to the parts per trillion (ppt) range and beyond. Typically, the sample is a liquid solution or suspension and is supplied by a nebulizer in the form of an aerosol in a carrier gas; generally argon or sometimes helium. The nebulized sample passes into a plasma torch, which typically comprises a number of concentric tubes forming respective channels and is surrounded towards the downstream end by a helical induction coil. A plasma gas, typically argon, flows in the outer channel and an electric discharge is applied to it, to ionize some of the plasma gas. A radio frequency electric current is supplied to the torch coil and the resulting alternating magnetic field causes the free electrons to be accelerated to bring about further ionization of the plasma gas. This process continues until a steady plasma state is achieved, at temperatures typically between 5,000K and 10,000K. The carrier gas and nebulized sample flow through the central torch channel and pass into the central region of the plasma, where the temperature is high enough to cause atomization and then ionization of the sample.
The sample ions in the plasma next need to be formed into an ion beam, for ion separation and detection by the mass spectrometer, which may be provided by a quadrupole mass analyser, a magnetic and/or electric sector analyser, a time-of-flight analyser, or an ion trap analyser, among others. This typically involves a number of stages of pressure reduction, extraction of the ions from the plasma and ion beam formation, and may include a collision/reaction cell stage for removing potentially interfering ions.
The first stage of pressure reduction is achieved by sampling the plasma through a first aperture in a vacuum interface, typically provided by a sampling cone having an apertured tip of inner diameter 0.5 to 1.5 mm. The sampled plasma expands downstream of the sampling cone, into an evacuated expansion chamber. The central portion of the expanding plasma then passes through a second aperture, provided by a skimmer cone, into a second evacuation chamber having a higher degree of vacuum. As the plasma expands through the skimmer cone, its density reduces sufficiently to allow extraction of the ions to form an ion beam, using strong electric fields generated by ion lenses downstream of the skimmer cone. The resulting ion beam may be deflected and/or guided onwards towards the mass spectrometer by one or more ion deflectors, ion lenses, and/or ion guides, which may operate with static or time-varying fields.
As mentioned, a collision/reaction cell may be provided upstream of the mass spectrometer, to remove potentially interfering ions from the ion beam. These are typically argon-based ions (such as Ar+, Ar2+, ArO+), but may include others, such as ionized hydrocarbons, metal oxides or metal hydroxides. The collision/reaction cell promotes ion-neutral collisions/reactions, whereby the unwanted molecular ions (and Ar+) are preferentially neutralized and pumped away along with other neutral gas components, or dissociated into ions of lower mass-to-charge ratios (m/z) and rejected in a downstream m/z discriminating stage. U.S. Pat. Nos. 7,230,232 and 7,119,330 provide examples of collision/reaction cells used in ICP-MS.
The ICP-MS instrument should preferably satisfy a number of analytical requirements, including high transmission, high stability, low influence from the sample matrix (the bulk composition of the sample, including, for example, water, organic compounds, acids, dissolved solids, and salts) in the plasma, and low throughput of oxide ions or doubly charged ions, etc. These parameters can be highly dependent upon the geometry and construction of both the sampling cone and the skimmer cone, as well as subsequent ion optics.
In view of the increasingly routine use of ICP-MS, the throughput of the instrument has become one of the most important parameters. The need for maintenance, cleaning and/or replacement of parts can reduce the working time of an instrument and thereby affect its throughput. This parameter depends strongly on memory effects caused by the deposition of material from previous samples, along the whole length of the instrument from sample input to detector, but in particular on the glassware of the plasma torch and on the inner and outer surfaces of the sampling cone and of the skimmer cone. The effect on the skimmer cone becomes more significant in instruments using more enclosed or elongated skimmer cones, as, for example, in U.S. Pat. Nos. 7,119,330 and 7,872,227 and Thermo Fisher Scientific Technical Note Nr. 40705.
It would therefore be desirable to provide a way of either reducing such deposition, or reducing the effect of such deposition, on the instrument so that the resulting loss of throughput may be reduced. The invention aims to address the above and other objectives by providing an improved or alternative skimmer cone apparatus and method.